In 51 patients, the cause for pleural effusion remained indeterminate immediately after thoracotomy. Thirty-one (60.8%) had no recurrence of the effusion, and no cause became apparent during a follow-up period of from 1 1/2 to 15 years. Two patients (3.9%) died relatively soon after thoracotomy, but death was not clearly related to the pleural effusion. In 18 patients (35.3%), the cause of the effusion became apparent from 12 days to 6 years after thoracotomy. In 13 of these 18 patients, malignancy (6 patients with lymphoma, 4 with malignant pleural mesothelioma, and 3 with other malignancy) was ultimately diagnosed. In 3 of the 18 patients, the ultimate diagnosis was a collagen-vascular disease, and 1 patient each had the yellow-nail syndrome and mitral stenosis.